xfs: factor out log buffer writing from xlog_sync

Replace the not very useful xlog_bdstrat wrapper with a new version that
that takes care of all the common logic for writing log buffers.  Use
the opportunity to avoid overloading the buffer address with the log
relative address, and to shed the unused return value.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>